Microbial community dynamics and extracellular enzyme activities in modern and ancient carbon dominated waters in northeast Siberia

Abstract

Permafrost thaw in the arctic as a result of climate change, leads to the transport of organic matter from terrestrial to fluvial networks. Part of this organic matter is mobilized from Pleistocene aged Yedoma permafrost and is highly labile. The degradation of this organic matter can enhance the positive feedback between permafrost melt and the climate system. Therefore it is important to understand the processes involved in its breakdown. Here microbial communities and potential extracellular enzyme activities in aquatic environments dominated by ancient and modern carbon in northeast Siberia were compared. A seven-day incubation experiment was done to assess the change in microbial communities and enzyme activities during the degradation of organic matter in ancient organic matter and modern organic matter waters. Bacterial and Archaeal communities were assessed using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S gene fragments. The activities of three hydrolytic enzymes were measured using fluorescent substrates in high throughput microplate assays. The activity of phenol oxidase was measured in the same way using colorimetric microplate assays. Some differences in microbial richness and diversity were found as well as diverse bacterial communities between the compared sites. Also, the particle associated bacteria showed a different response to incubation than the free living bacteria. However due to methodological constraints, no hard conclusions could be drawn from the PCR-DGGE analysis. In contrast to earlier studies on dissolved organic matter (Mann et al. (2013) and Vonk et al. (2013b), high phenol oxidase activities as well as hydrolytic enzyme activities were found in ancient permafrost degradations. For the Kolyma river, no phenol oxidase activities were detected, while hydrolytic enzyme activities increase, indicating nutrient limitation. Y3 showed no phenol oxidase activities and a decrease in hydrolytic enzyme activities. In this study enzyme activities were in fact assessed on particulate matter instead of dissolver organic matter. The particulate fraction in these environments might thus have a different phenolic content than the dissolved fraction. There also the metabolic mechanisms involved in that breakdown of the particulate organic material might be different than for the dissolved organic material.